The purpose of this research study is to examine whether the effect of the drug "Methylephenidate" that has been used in the therapy of Parkinson's disease is different in patients with Parkinson's disease compared with individuals without this disease. The study will test whether differences in feeling that these drugs can induce in normal individuals may or may not be present in patients with Parkinson's disease. Preliminary testing was initiated in 1989, the final phase of this study is now underway.